1. Field of the Invention
The present invention generally relates to a finely perforated shadow mask in a color cathode ray tube and, more particularly, to a method of forming an electron reflecting layer on the finely perforated shadow mask for facilitating reflection of electron beams thereby to minimize any possible thermal deformation of the finely perforated shadow mask.
2. Description of the Prior Art
As is well known to those skilled in the art, the color cathode ray tube generally comprises a highly evacuated envelope including a funnel section generally tapering in one direction, a generally cylindrical neck section closed at a rear end and continued at a front end to the reduced diameter end of the funnel section and accommodating therein a color electron gun assembly comprised of red, green and blue electron guns, a faceplate having its periphery sealed to the large diameter end of the funnel section and also having a screen plate. The screen plate has an inner surface deposited with a multiplicity of generally vertically running stripes of cyclically distributed, elemental color phosphors, for example, red, green and blue phosphors, which surface confronts the electron gun assembly. The evacuated envelope also includes a finely perforated shadow mask lying perpendicular to the longitudinal axis of the evacuated envelope and having a multiplicity of fine slits each provided for one group of the three elemental color phosphors on the inner surface cf the screen plate, which mask is housed within the envelope and positioned in the vicinity of and in parallel relationship with the phosphor deposited inner surface of the screen plate. The groups of the three elemental color phosphors on the inner surface of the screen plate are so defined and so positioned as to optoelectrically correspond to the slits defined in the perforated shadow mask.
FIG. 1 of the accompanying drawings schematically illustrates a longitudinal sectional view of the well known color cathode ray tube, wherein reference numeral 1 represents the highly evacuated envelope, reference numeral 2 represents the electron gun assembly, reference numeral 3 represents the finely perforated shadow mask, reference numeral 4 represents the screen plate, reference numeral 5 represents the phosphor deposited inner surface of the screen plate 4, reference numeral 6 represents a deflection yoke assembly mounted on the neck section in the vicinity of the funnel section and comprised of two pairs of electromagnetic coils disposed at right angles to each other on the neck section, reference numeral 7 represents the slits defined in the perforated shadow mask 3, and reference numeral 8 represents electron beams emitted from the electron gun assembly 2.
The color cathode ray tube of the above described construction operates in the following manner. Three electron beams 8 of different colors emitted from the color electron gun assembly 2 travel towards the phosphor deposited inner surface 5 of the screen plate 4. On their course towards the screen plate 4, the electron beams 8 are passed through a deflection magnetic field developed by the deflection yoke assembly 6 so that the electron beams can sweep across the phosphor deposited inner surface 5 of the screen plate 4 from left to right and from top to bottom while successively exciting the elemental color phosphor deposits on the inner surface 5 of the screen plate 4. After the passage of the color electron beams 8 through the deflection magnetic field, the color electron beams 8 pass through selected ones of the slits 7 in the perforated shadow mask 3. As is well known to those skilled in the art, the perforated shadow mask 3 serves as a color selection electrode operable to allow any single electron beam to impinge only upon the phosphor deposits of a particular one of the three elemental colors as the perforated shadow mask 3 is so uniquely positioned relative to the phosphor deposited inner surface 5 of the screen plate 4 as to permit any one group of the three elemental color phosphors on the inner surface 5 to correspond to the associated slits 7 in the perforated shadow mask 3.
In this well known color cathode ray tube, it is also well known that about 80% of the color electron beams produced by the color electron gun assembly 2 is said to impinge upon the perforated shadow mask 3 without passing through the slits 7 and will therefore not reach the phosphor deposited inner surface 5 of the screen plate 4. It is this quantity of the color electron beams that applies heat energies to the perforated shadow mask 3 thereby to cause the latter to be undesirably heated. Once the perforated shadow mask 3 is so heated, the perforated shadow mask undergoes an undesirable thermal expansion that results in a thermal deformation of the perforated shadow mask 3 generally known as doming. The doming phenomenon is known as a cause of a deviation in positional relationship between the elemental color phosphor deposits on the inner surface 5 of the screen plate 4 and the patterned slits 7 in the perforated shadow mask 3, that is, a cause of mislanding of the electron beams upon the phosphor deposited inner surface 5.
As a method of substantially alleviating the above discussed problem, the Japanese Patent Publication No. 61-6969, published in 1986, and U.S. Pat. No. 4,810,927 issued Mar. 7, 1989, and assigned to the same assignee of the present invention, discloses the formation of an electron reflecting layer 9 on the perforated shadow mask 3 as shown in FIG. 2 which shows a fragmentary enlarged view of a portion of the perforated shadow mask 3. According to any one of those references, the electron reflecting layer 9 is made of a material having a higher reflectivity to the incident electron beams than that of a material used to form the perforated shadow mask 3.
The Japanese Patent Publication No. 60-14459, published in 1985, and U.S. Pat. No. 4,442,376, disclose the formation of the electron reflecting layer 9 by spraying a suspension, containing a quantity of powdered heavy metal having its atomic number of 70 or greater, onto the perforated shadow mask 3 while air is drawn through the slits 7 in the perforated shadow mask 3 from a location on one side of the perforated shadow mask 3 adjacent the phosphor deposited inner surface of the screen plate 4.
One example of the heavy metal whose atomic number is 70 or greater may include a bismuth oxide. In practice, the bismuth oxide is employed in the form of powdered particles of not greater than 1 .mu.m in particle size which are added to an aqueous solution of water glass to provide the suspension. To form the electron reflecting layer 9, a spraying technique is employed to apply the suspension containing the powdered bismuth oxide over the perforated shadow mask 3.
However, it has generally been considered difficult to realize a favorable method of forming the electron reflecting layer on the perforated shadow mask. Specifically, the application of the suspension containing the powdered bismuth oxide by the use of the spraying technique requires the content of the bismuth oxide to be properly selected to a required value in dependence on the particle size of the powdered bismuth oxide used. Unless the content of the powdered bismuth oxide is selected properly, the viscosity of the resultant suspension tends to become excessively high or low so enough as to result in a clogging of the particles within a spraying gun or as to result in a difficulty in spraying the suspension uniformly over the perforated shadow mask.
Also, if the content of the water glass used in the suspension as a binder is increased in an attempt to increase the bondability of the applied bismuth coating, that is, the eventually formed electron reflecting layer, to the surface of the perforated shadow mask, an increased amount of carbon dioxide tends to be adsorbed by the water glass, resulting in a considerable reduction of the lifetime of the perforated shadow mask in the color cathode ray tube. On the other hand, if the content of the water glass in the suspension is short of the required quantity, an insufficient bondability will be attained between the electron reflecting layer and the perforated shadow mask and the electron reflecting layer will be readily peeled off from the perforate shadow mask.